Advancing age, as well as injuries, can lead to changes in the various bones, discs, joints and ligaments of the body. In particular, these changes can manifest themselves in the form of damage or degeneration of an intervertebral disc, the result of which is mild to severe chronic back pain. Intervertebral discs serve as “shock” absorbers for the spinal column, absorbing pressure delivered to the spinal column. Additionally, they maintain the proper anatomical separation between two adjacent vertebra. This separation is necessary for allowing both the afferent and efferent nerves to exit and enter, respectively, the spinal column.
Treatment for a diseased or damaged disc can involve the removal of the affected disc and subsequent fusion of the opposing vertebra to one another. Spinal fusion consists of fusing the adjacent vertebrae through the disc space (the space previously occupied by the spinal disc interposed between the adjacent vertebral bodies). Typically, a fusion cage and/or bone graft is placed into the disc space to position the vertebrae apart so as to create more space for the nerves, to restore the angular relationship between the adjacent vertebrae to be fused, and to provide for material that can participate in and promote the fusion process.
More recently, spinal prostheses have been developed that allow one or more degrees of freedom between the adjacent vertebrae, thereby restoring function to the vertebrae. One well known spinal prosthesis is the CHARITÉ® produced by DePuy Spine, Inc. of Raynham, Mass., which includes two metal endplates and a center polyethylene core. The center core includes superior and inferior spherical bearing surfaces that are received within concave surfaces formed in the superior and inferior endplates. Once implanted, the prosthesis will allow multiple degrees of freedom between the endplates of the adjacent vertebrae. Other prostheses are known having a semi-constrained configuration, allowing minimal degrees of freedom between the endplates of the vertebrae, or a constrained configuration, allowing only one degree of freedom between the endplates of the vertebrae.
The particular level of laxity or constraint needed can vary depending on the patient's condition. For instance, a patient with symptomatic or suspect disc at the neighboring level may be better served with a less constrained device, like the CHARITÉ®. Conversely, a surgeon may select to implant a prosthesis with less laxity to limit motion due to the patient's facet condition at the operative level, as might be the case for an older patient with less activity.
While prostheses having various degrees of freedom are known in the art, no one prosthesis allows a surgeon with the flexibility to select the degree of constraint either pre- or intra-operatively based upon the patients sagittal and/or coronal alignment, facet and/or annulus condition, neighboring level condition, as well as various other factors. Accordingly, there remains a need for a disc prosthesis system which allows for greater modularity, and in particular which allows a surgeon to construct a prosthesis having a desired degree of freedom.